Magnetic recording mediums such as magnetic tapes are commonly prepared by applying to a support a magnetic coating composition comprising a magnetic powder, a binder (or binder resin) and so forth, followed by drying.
In recent years, the magnetic recording mediums prepared in this way have made advance in their uses and commercialization in more and more fields, and accordingly have become required to have electromagnetic conversion characteristics, physical properties and other characteristics which are far superior to those of conventional ones. For this reason, various studied and improvements have been made also in respect of binders used in magnetic materials.
That is to say, as binders used for magnetic recording mediums, a cellulose derivative, a vinyl chloride/vinyl acetate copolymer, a polyurethane resin, an acrylic resin, copolymers of these, a vinylidene chloride resin and a copolymer thereof, an epoxy resin, a phenoxy resin, a polyester and so forth are used alone or in combination of several kinds of components. In particular, a typical example is a combination of a polyurethane resin with a vinyl chloride/vinyl acetate copolymer.
Even the combination of these components, however, can bring about no satisfactory results in respect of dispersibility of magnetic powder, and surface properties, wear resistance and durability that are required in a magnetic coating film. Hence, other required performances of a medium are often restricted. In addition, the S/N ratios become higher and magnetic powder is more densely packed in a medium along the recent tendency that magnetic powder is comprised of finer particles. This on the other hand brings about the deterioration of strength of a coating film and makes it impossible to sufficiently cope with troubles o defects such as dropout and edge break when a medium is formed into a tape.